Document WO-2008/155,268 describes an internal-combustion engine with two supercharging stages, allowing to further increase compression of the air fed into the combustion chamber while improving the efficiency of this engine and reducing its fuel consumption.
More precisely, these two stages are arranged in series in relation to one another and they consist of two turbochargers, a high-pressure (HP) turbocharger and a low-pressure (LP) turbocharger. The HP turbocharger is more particularly suited for low flow rates whereas the LP turbocharger is suited for higher flow rates.
These turbochargers consist each of an expansion turbine connected by a drive shaft to a compressor. These two turbochargers are so laid out that the turbine of the HP turbocharger, supplied with the exhaust gas taken from the engine exhaust pipe, supplies in turn the turbine of the LP turbocharger while the compressor of the LP turbocharger supplies the compressor of the HP turbocharger.
As better described in the aforementioned document, a burnt gas recirculation circuit referred to as EGR circuit is provided. This circuit essentially comprises a line, referred to as EGR line, allowing exhaust gas to be introduced from a branch connection on the exhaust pipe of this engine (generally at the exhaust manifold outlet) to a connection point at the intake thereof (more precisely to the intake distributor).
This EGR circuit thus allows exhaust gas to be added to the fuel mixture, notably in order to decrease the production of nitrogen oxides (NOx) by reducing the combustion temperature of this mixture.
An additional gas purification means is also housed upstream from this EGR line and downstream from the exhaust. This purification means, referred to as precatalyst hereafter, adds further to the main exhaust gas purification device that all engines are usually equipped with in form of a catalyst.
In order to be able to treat the pollutants contained in the exhaust gas efficiently, this main catalyst, which is initially at ambient temperature, has to be heated by the exhaust gas flowing therethrough until it reaches a sufficient temperature level for its light-off.
In order to minimize the discharge of pollutants to the atmosphere, the precatalyst is intended to reach more rapidly its light-off temperature than the main catalyst considering that it is located closer to the exhaust outlet. This precatalyst thus allows part of the pollutants of these gases to be treated more rapidly.
More particularly, this precatalyst is advantageously an oxidation precatalyst allowing the HC (unburnt hydrocarbons) and the CO (carbon monoxide) contained in the exhaust gas to be treated.
However, this precatalyst, when traversed by high exhaust gas flow rates, generates a significant pressure drop with the major drawback of creating a back pressure that hinders discharge of the exhaust gas out of the engine cylinders.
In order to avoid this, and as better described in the aforementioned document, it is necessary for the precatalyst to be arranged downstream from the bypass line the HP turbine is fitted with, and which allows this turbine to be bypassed for gases with high flow rates.
No pressure drop is therefore created by this precatalyst when the LP turbocharger is used alone.
Another drawback lies in the fact that this precatalyst is not arranged in such a way that the recirculated exhaust gas is also purified prior to being fed again to the intake. This is all the more prejudicial since removal of the HC and CO contained in this gas allows, on the one hand, to facilitate load control and, on the other hand, to reduce fouling of the various parts of the engine scavenged by this recirculated exhaust gas.
The present invention aims to overcome the aforementioned drawbacks by means of a system that provides the same functionalities of the supercharging system while allowing the exhaust gas of the EGR circuit to be purified.